Ion exchange membranes and dissolved gas sensors

ABSTRACT

Ion exchange membranes for use in sensors that measure dissolved gases are described. Sensors constructed using the disclosed membranes are able to maintain electrolyte conditions within the electrolyte volume so as to have greatly extended and more stable lifetimes than sensors of similar construction and electrolyte volume constructed with standard membranes.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of International Application No.PCT/CA2002/000750, filed May 21, 2002, the disclosure of which isincorporated by reference herein.

FIELD

The invention relates to ion exchange membranes and dissolved gassensors.

BACKGROUND

Measurement of dissolved gases is important in many fields, includingmedicine, food science and environmental science. One type of sensordeveloped to measure dissolved gases is the Clark cell. Clark cells areused to detect gases that are readily reduced or oxidized, such ashydrogen sulfide, NO, NO₂, CO, Cl₂ and O₂. These sensors consist of agas permeable membrane enclosing an electrolyte and working andreference electrodes in contact with the electrolyte (see, for example,Janata, J., Principles of Chemical Sensors, Plenum Publishing, 1991 andPolarographic Oxygen Sensors, Chapter 4, Gnaiger, E. and Forstner, H.(Eds.), Springer-Verlag, 1983). Gases pass through the membrane bydiffusion, and are reduced or oxidized at the working electrode tocreate a detectable current flow.

The stability and reliability of Clark cells depends on many factors,but in particular there is a limitation on sensor lifetime imposed bythe amount of electrolyte within the Clark cell. This problem has beencentral to Clark cells since their inception and is mentioned in manyearlier patents (see, for example, U.S. Pat. No. 5,212,050). Sinceoxidation and reduction processes consume components of the electrolyte,Clark cells are inherently prone to instability and limited lifetime dueto exhaustion of the electrolyte. This problem is particularly acute forsmall electrodes (microelectrodes) and there have been many attempts inthe past to address this shortcoming mechanically. For example,increasing the electrolyte volume and providing a means to replenish theelectrolyte are two approaches. Mechanical solutions, however, tend toincrease the complexity of the sensor (see, for example, European PatentNo. EP 0496521). For these reasons, Clark cells are typically expensiveto construct and require frequent maintenance and calibration.

SUMMARY

Gas-permeable membranes comprising an anion exchanger, such as aguanidinium salt, and sensors made with such membranes are described.The membranes exhibit selective permeability for the gas to be analyzedand an ion exchange capacity that allows discharge of the ionic productsof the redox reaction used to detect the gas and replenishment ofelectrolyte components consumed in the redox reaction from the samplebeing measured. The membranes alleviate problems associated withexhaustion of the sensor electrolyte and allow amperometric sensors tofunction in a stable manner for a much longer period of time than asensor that must rely on only the ions present in the originalelectrolyte volume. The disclosed, membranes therefore permitconstruction of very small and long lived, stable sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing one embodiment of an oxygen sensorillustrating a method for replenishing the interior electrolyte andpreventing build-up of redox products using ion exchange membranes.

FIG. 2 is a graph showing the output of a disclosed sensor as a functionof the current measured by a commercial dissolved oxygen meter.

FIG. 3 is a graph showing the response of a disclosed sensor in responseto an abrupt change in dissolved oxygen concentration.

FIG. 4 is a graph showing the output current of a disclosed sensor overa six-hour period.

FIG. 5 is a graph showing the output current of a matched pair ofsensors under forcing conditions as a function of time, one sensorhaving a membrane according to the disclosure, the other without.

FIG. 6 is a graph showing the output current of a matched pair ofsensors under forcing conditions as a function of the total chargetransferred, one sensor having a membrane according to the disclosure,the other without.

FIG. 7 is a cross-sectional schematic diagram showing one embodiment ofa disclosed dissolved gas sensor constructed on a printed circuit board.

FIG. 8 is a schematic diagram showing a top-view of one embodiment of adisclosed dissolved gas sensor constructed on a printed circuit board.

FIG. 9 is a graph showing the sensor output versus dissolved oxygenconcentration for a printed circuit board sensor according to oneembodiment of the disclosure.

FIG. 10 is a graph showing the response characteristics of a printedcircuit board sensor according to one embodiment of the disclosure incomparison to a commercial Clark cell sensor.

DETAILED DESCRIPTION

Although the performance of sensors for all readily reducible andoxidizable gases may be improved using the disclosed membranes and theprinciple on which they operate, the following description focuses ondissolved oxygen sensors. A Clark cell for oxygen detection consists ofan inert cathode, typically gold or platinum, and a reversible anode,such as silver/silver chloride, within an electrolyte volume that isseparated from the sample by an oxygen permeable membrane. As oxygendiffuses into the electrolyte volume, oxygen reduction at the cathodeproduces hydroxide ions that increase in concentration in the vicinityof the cathode. As current flows, there is a concomitant depletion ofchloride ions in the vicinity of the anode. These concentration changesalter the stability of the sensor and, as the electrolyte is consumed,will ultimately inactivate the sensor.

The disclosed membranes alleviate these problems as, diagrammed inFIG. 1. An anion exchanger is included within the gas-permeable membraneto remove hydroxide ions from and transport chloride ions into theelectrolyte volume of the cell, ensuring a longer working lifetime andgreater stability of the sensor over time. The external solution beinganalyzed acts as a sink for hydroxide ions and a source of chloride ionsin this system. Thus, the medium in which the gas is being measured needonly have sufficient chloride ions for the exchange mechanism to operateand sufficient capacity to absorb hydroxide ions. Examples of such mediainclude seawater, foodstuffs, and biological fluids.

For oxygen sensors, any anion exchanger that can shuttle hydroxide andchloride ions through the gas permeable membrane may be utilized (forexample, cationic species such as cationic metal complexes, guanidiniumsalts and ammonium salts, including quaternary ammonium salts). However,in particular embodiments the ion exchanger is a guanidinium salt havingthe formula:

In general, R₁, R₂, R₃, R₄, R₅, and R₆ may be independently chosen toimpart an affinity of the guanidinium salt for the membrane phase and X⁻is any anion, for example, any type of halide ion. In particulardisclosed embodiments, R₁, R₂, R₃, R₄, R₅, and R₆ may be independentlyselected from the group consisting of hydrogen, alkyl, substitutedalkyl, cycloalkyl, substituted cycloalkyl, alkenyl, substituted alkenyl,cycloalkenyl, substituted cycloalkenyl, alkynyl, aryl, substituted aryl,heteroaryl and substituted heteroaryl. More particularly R₁, R₂, R₃, R₄,R₅, and R₆ may be independently selected from the group consisting ofhydrogen, C1-30 alkyl, and aryl (for example, phenyl or napthyl). Inparticular embodiments X⁻ may be selected from the group consisting ofchloride, bromide, fluoride, iodide, hydroxide, acetate, carbonate,sulfate, and nitrate and combinations thereof.

The gas permeable membrane may be any one of several types. One type ofmembrane is a supported liquid membrane (SLM) (see, for example, LiquidMembranes: Theory and Applications, Noble and Way (Eds.), ACS SymposiumSeries 347, American Chemical Society, 1987). In such membranes, aporous polymer support, such as microporous polyalkylene, includingpolytetrafluoroethylene, polyethylene or polypropylene, is imbibed witha solvent containing the ion exchanger. Suitable solvents includehigh-boiling solvents such as ortho-nitrophenyl ether, dioctyl adipateand others described below.

Another type of gas-permeable membrane is a polymer, such as highmolecular weight poly(vinyl chloride) (PVC), silicon rubber or acellulose ester (e.g. cellulose acetate), that is plasticized with asolvent that also serves to dissolve the ion exchanger. Suitablesolvents include, without limitations, adipate esters (e.g. dioctyladipate and diisononyladipate), sebacate esters (e.g. dioctyl sebacateand bis(2-ethylhexyl)sebacate), phthalate esters (e.g. dioctylphthalate,bis(2-ethylhexyl)phthalate, and butyl hexyl phthalate), glycol esters(e.g. diethyleneglycol dibenzoate and dipropyleneglycol dibenzoate), lowvolatility ethers (e.g. ortho-nitrophenyl octyl ether), trimellitic acidesters (e.g. tris(2-ethylhexyl)trimellitate), phosphate triesters (e.g.isodecyl diphenylphosphate, and tert-butylphenyl diphenylphosphate),chlorinated paraffins (e.g. chlorowax and flexchlor), and mixturesthereof. Plasticized polymer membranes may be cast as a solution of thepolymer and plasticizer in a volatile solvent such as tetrahydrofuran ortrifluoroethanol.

Yet another type of gas-permeable membrane can be formed by modificationof a polymer backbone to provide ion exchange groups directly linked tothe polymer backbone. Any of the types of membranes described above maybe fabricated with 0.1-10 wt %, for example, 1-5 wt %, of an added ionexchanger (e.g. the guanidinium salts described above). Other types ofgas-permeable membranes, known or yet to be discovered, may benefit fromadding ion exchangers as described herein.

The disclosed membranes can be used with a variety of constructionmethods to provide amperometric sensors with any number of geometries.In addition, the disclosed membranes may be applied in place of existinggas-permeable membranes to improve the lifetime and stability of anexisting sensor. In one embodiment, the disclosed membranes are appliedto “solid state” micro-fabricated sensor systems to more fully takeadvantage of the low electrolyte volumes required (for example,electrolyte volumes of less than 50 μL, less than 25 μL, less than 10μL, less than 5 μL or less than 1 μL are possible). Working embodimentstypically included about 5 μL. For instance, the membrane may be cast inplace over a suitable dry salt such as NaCl or KCl that has been placedover a pair of electrodes so as to form a “solid state” sensor. Such asensor may be stored “dry” and “wetted” before use (see, for example,U.S. Pat. No. 4,933,048).

Suitable inert cathode materials for sensors incorporating the membranesof the disclosure may be selected from the group consisting of gold,platinum, silver, palladium, iridium, rhodium, ruthenium, osmium andalloys thereof. Suitable reversible anode materials may be selected fromthe group consisting of silver/silver halide (e.g. silver/silverchloride, silver/silver bromide, silver/silver fluoride, andsilver/silver iodide), lead/lead sulfate, silver/silver oxide-hydroxideand lead/lead oxide-hydroxide. Suitable electrolyte salts may beselected from the group consisting of water-soluble cation/anion pairsformed from the cations of Na, K, Cs, Rb, Li, Mg, Ca, Ag, Zn, and Pb andmixtures thereof, and the anions of F, Cl, Br and I and mixturesthereof. The following anions also can be used in combination with thelisted cations: acetate, perchlorate, hydroxide, carbonate, sulfate, andnitrate and mixtures thereof. In working embodiments, the electrolytesalts used have been alkali metal halides and mixtures thereof, such asKCl and NaCl and mixtures thereof.

Suitable insulating substrates for making sensors include ceramics (e.g.alumina) and glass. Plastics may also be used as substrates for makingsensors, provided the plastic is impervious to the gas being measured bythe sensor.

The disclosed membranes offer advantages in addition to improvedlongevity and stability. For example, because the membranes permitconstruction of Clark-type sensors with very small electrolyte volumes,the sensors are able to withstand cycling between low and high pressurebecause gases apparently diffuse out of the sensors faster than formingbubbles. In some embodiments, the sensors are able to withstand steamsterilization and are thus suitable for use in the food industry.

Thus, in another aspect an an amperometric gas sensor is disclosed,where the sensor includes an anode and a cathode deposited on agas-impervious substrate. An electrolyte between the anode and thecathode conducts current between the two electrodes. The anode, cathodeand the electrolyte are covered by a gas-permeable membrane comprising aguanidinium salt. The sensor can further include a well surrounding theanode and the cathode. The gas permeable membrane covers the well andthus defines an electrolyte volume within the well. In a particularembodiment, the well is formed by a hole in a laminate material that isplaced over the electrodes. Optionally, the sensor can also include aguard ring deposited on the gas-impervious substrate. In this case, thegas permeable membrane also covers the guard ring.

In some embodiments, the electrolyte is a fluid containing a dissolvedsalt, a solid salt, or a hydrogel including an electrolyte. For example,the electrolyte can be a Group I metal halide such as KCl, NaCl or amixture thereof Suitable hydrogels include cross-linked acrylates,methyl methacrylates, methacrylates, hydryxalkyl acrylates,hydroxyalkyl(meth)acrylates, acrylamides, silicone hydrogels, gelatin,cellulose nitrate, cellulose, agar, and agarose and combinationsthereof.

The gas-permeable membrane of the sensor (that includes a guanidiniumsalt) can be a supported liquid membrane, for example, a porous supportpolymer comprising a solvent such as a polytetrafluoroethylene membraneand a solvent selected from the group consisting of o-nitrophenyl octylether and dioctyl adipate, and mixtures thereof. In other embodiments,the supported liquid membrane includes a plasticized polymer, forexample, poly(vinyl chloride) and a phthalate plasticizer such as a highmolecular weight poly(vinyl chloride) plasticized with a solventselected from the group consisting of o-nitrophenyl octyl ether anddioctyl adipate, and mixtures thereof. In particular embodiments, theguanidinium salt is not covalently bonded to the membrane. In others,the guanidinium salt comprises 1% to 5% of the membrane.

In yet anther aspect a method of making an amperometric gas sensor isdisclosed. The method includes depositing an anode and a cathode onto agas-impervious substrate, placing an electrolyte between the anode andthe cathode, and covering the anode, the cathode and the electrolytewith a gas-permeable membrane. The gas-permeable membrane includes aguanidinium salt. In particular embodiments, the anode and cathode aredeposited onto the substrate by printing the anode and cathode onto thesubstrate using a method for printing circuit boards. A guard ring alsocan be deposited onto the substrate.

The method of making the sensor can further include forming a wellaround the anode and the cathode and covering the well with the membraneto define an electrolyte volume (for example, a volume from 5 μL to 50μL). An electrolyte can be added to the well to place the electrolytebetween the anode and cathode. The electrolyte can be added to the wellas a solution (for example, an aqueous solution), which can optionallybe allowed to dry. Alternatively, the electrolyte can be added to thewell by forming a hydrogel in the well, for example, a hydrogel selectedfrom the group consisting of cross-linked acrylates, methylmethacrylates, methacrylates, hydryxalkyl acrylates,hydroxyalkyl(meth)acrylates, acrylamides, silicone hydrogels, gelatin,cellulose nitrate, cellulose, agar, and agarose and combinationsthereof.

In one embodiment, a well around the anode and the cathode is formed byplacing a laminating material having a hole onto the substrate such thatthe hole is disposed over the anode and cathode. The well can becovered, for example, by deposting a plasticized PVC membrane materialdissolved in a volatile solvent over the well, and optionally, firstcovering the well with a layer of microporous cellulose acetate and thendepositing the PVC membrane material onto the microporous celluloseacetate. In other embodiments, the well can be formed in thegas-impervious substrate itself. For example, the well can be etched ormachined (such as micromachined) into the substrate. Alternatively,separate layers of substrate material can be laminated together to formthe well, where a first layer having the electrodes is laminated to asecond layer and the second layer has a hole machined or etched (eitherbefore or after lamination) at a position disposed over the electrodeswhen the layers are laminated together.

In another aspect, an amperometric gas sensor formed on a printedcircuit board is disclosed. In this embodiment, an inert cathode and areversible anode are patterned on a gas-imperviouselectrically-insulating substrate, and an electrolyte between thecathode and the anode is included. A gas-permeable membrane covering thecathode, the anode and the electrolyte is sealed at its outer edges toprevent communication between the electrolyte and a medium in which agas is sensed except through the membrane.

In yet another aspect, an amperometric gas sensor is disclosed includingan inert cathode and a reversible anode printed on a gas-imperviouscircuit board substrate with a well surrounding the cathode and theanode. The well contains a hydrogel comprising an electrolyte and agas-permeable membrane including a guanidinium salt covers the well. Inparticular embodiments, the inert cathode is selected from the groupconsisting of gold, platinum, silver, palladium, iridium, rhodium,ruthenium, osmium and alloys thereof and the reversible anode comprisesa material selected from the group consisting of silver/silver halide,lead/lead sulfate, silver/silver oxide hydroxide and lead/leadoxide-hydroxide. In more particular embodiments, the inert cathodecomprises a material selected from the group comprising of gold andplatinum and the reversible anode is an Ag/AgCl electrode. In otherparticular embodiments, the hydrogel is selected from the groupconsisting of cross-linked acrylates, methyl methacrylates,methacrylates, hydryxalkyl acrylates, hydroxyalkyl(meth)acrylates,acrylamides, silicone hydrogels, gelatin, cellulose nitrate, cellulose,agar, and agarose, and combinations thereof.

The following examples are provided to aid in the understanding of thedisclosure and are not meant to limit the scope of the invention.

EXAMPLE 1 Amperometric Sensors With a Solid Electrolyte Salt

The membranes described herein may be used to construct a sensor with asolid electrolyte. Such sensors may be constructed as follows:

-   -   1) An anode and cathode are placed upon a gas-impervious,        electrically insulating substrate using standard printed circuit        board techniques (see, for example, U.S. Pat. No. 4,534,356). In        some embodiments a third electrode or “guard ring” also may be        formed on the substrate in the same manner as the other        electrodes. For example, the electrodes may be formed using an        appropriate mask (e.g. photoresist) and metal slurries (e.g.        metal slurries provided by Englehard, E. I. duPont de Nemours,        or Johnson Matthey). If one of the electrodes is to be a        silver/silver halide electrode it may be formed by depositing        silver on the surface of the substrate and then halogenating the        silver electrode by electrochemical techniques. For example, the        silver electrode may be halogenated by chloridation using a        solution of 1% sodium chloride. Chloridation can be achieved,        for example, at 3.5 V at low current for a short period of time,        such as 10 minutes. Electrodes may also be formed by sputter        coating or applied as a thick or thin film. Any method, which        places the electrodes of the appropriate materials in the        appropriate position and proportion on a gas-impervious        electrically insulating surface, will suffice. Provision is made        to connect the electrodes to an electronics circuit capable of        providing the correct bias voltage and amplifying the resulting        signal. The electronics may be manufactured directly on the same        substrate as the electrodes to produce a “chip scale” (sensor on        a chip) sensor. The substrate also may be a flexible material,        provided the rest of the components are constructed in a fashion        that allows for flexion of the substrate.    -   2) A small quantity of electrolyte solution (for example 5        microlitres) is applied over the two electrodes and the solvent        is allowed to evaporate, leaving a coating of dry salt over the        surface of the electrodes.    -   3) Over this assembly a gas-permeable membrane of a formulation        described herein is applied such that it covers both electrodes        and the salt and is sealed on its outer edges to prevent        communication between the inner electrolyte and the outer medium        except by way of the membrane itself. The membrane may be “cast”        in place in liquid form and allowed to polymerize or it may be        pre-cast and applied to the substrate using a sealant or        mechanical device to seal the outer edges from the outside        medium.    -   4) Before operation, the sensor is placed in the liquid medium        in which gas readings are to be taken and allowed to “hydrate”        for a period of time. The hydration time varies depending upon        several factors, including temperature, pressure and the        chemical constituents of the medium. Working embodiments        achieved hydration in about 10 to 20 minutes. Once the sensor        has come into equilibrium with the environment, stable gas        concentration readings may be taken.

Any number of gas sensors may be constructed on a single substrate,along with additional sensors, such as pH sensors and temperaturesensors, to provide a compact sensor array.

EXAMPLE 2 Characterization of a Gas-Permeable Membrane Containing an IonExchanger

This example demonstrates the ion-exchange capacity of the disclosedmembranes and its implications for making stable, low-volume dissolvedoxygen (DO) electrodes. The advantage of the DO electrode is best seenunder “forcing” conditions in which electrode failure occurs in arelatively brief period (<24 hours). This can be achieved using alimited amount of electrolyte: an electrode producing a 0.2 μA currentwill consume the available chloride ions in 3 μL of 0.01 M NaCl in about4 hours.

A PVC/dioctyl adipate membrane comprising tetradecylguanidinium chloridewas tested with an experimental system that uses a measurement volume of0.35M NaCl at 12±0.5° C. and a set potential of 0.500±0.001V. The systemwas stirred from below and the solutions were open to the atmosphere, sodaily pressure variations could be seen in the longer data records. Theprobe uses a Pt cathode (˜0.5 mm diameter) at the center of a silverring anode (4 mm diameter, 1 mm thickness). The electrodes are bedded inepoxy and ground flat. The membranes mount directly on the flat surfaceholding the drop of electrolyte in place. Comparisons are based onmatched probes and parallel circuitry. The calibration, response time,and stability criteria are established with “unlimited” electrolyte,typically 5 μL of 0.1 M NaCl to give a working lifetime beforeelectrolyte exhaustion of at least 48 hours. The lifetime criterion isestablished using “forcing” conditions of limited electrolyte, typically5 μL of <0.01 M NaCl.

-   -   1) Calibration: For a thin membrane layer, the calibration will        be linear in dissolved oxygen concentration with a zero        intercept (one-point calibration). Conventional “thin” membranes        on the probe produce ˜0.05 μA/mgL⁻¹ DO or about 0.5 μA for an        oxygen saturated solution at the salinity and temperature noted        above. The disclosed membranes will produce similar amounts of        current at the same thickness. The fabrication procedures used        for a long-lived electrode produce a membrane ca. 150 μm thick.        Hence, the saturation current is somewhat lower (0.2 μA). FIG. 2        shows that the test electrode with the guanidinium salt as ion        exchanger shows a slightly curved calibration response and a        non-zero intercept with respect to a commercial Orion model 810.        The Orion meter loses its internal calibration (cell current        converted to ppm DO) after a period of about 20 minutes, but the        “raw” current values continue to be reported by the Orion        system.    -   2) Response time: This is defined as a time to respond to an        abrupt change from an oxygen saturated solution to another at        about 20% saturation and back again. The response time is        usually in terms of >95% total change in a certain number (x)        minutes. For a “thick” and aged electrode (beyond 1 equivalent        of electrolyte) this will typically be <5 minutes. Thinner        membranes respond more quickly. FIG. 3 shows the response time        function for a 2-day old electrode having a membrane according        to the disclosure.    -   3) Stability: This is the change, or drift, in the output signal        for a period where the calibration applies for an electrode        which is not electrolyte limited. It is usually set in terms of        drift per time e.g. <2% per hour measured over a time certain,        such as a 3-hour period. FIG. 4 shows a 6-hour period in which        the drift in the output signal is less than 0.06%. The variation        in the signal could be due to atmospheric variation in this        period. The digital noise is evident.    -   4) Lifetime: A conventional electrode will consume the available        electrolyte and eventually fail. The long-lived electrodes allow        ion exchange and therefore do not fail when the electrolyte runs        out. They do however, shift to a new current level which        balances the oxygen consumption and the anion exchange        processes. This is demonstrated by measuring the output signal        as a function of total charge transferred. FIG. 5 shows the        time-course for matched electrodes with and without ion        exchanger in the membrane. The same output data are given as a        function of total charge transferred in FIG. 6. Both electrodes        contain sufficient electrolyte to consume 4.8 mC of charge.        However, the conventional cell expireed before this theoretical        limit. The long-lived cell of the disclosure achieved a        relatively steady current about the theoretical limit of 5 mC        and thereafter maintained the oxygen consumption and ion        exchange in balance for a 3-day period.

EXAMPLE 3 Construction and Characterization of a Printed Circuit BoardDissolved Oxygen Sensor

The components of a printed circuit board (PCB) sensor constructed usingthe techniques described in Example 1 is shown in FIG. 7. FIG. 7 shows across-section of sensor (10) that includes substrate (20), cathode (30),such as a gold cathode, an anode (40), such as a silver/silver chloridereversible anode, optional guard ring (50), such as a silver guard ring,solid electrolyte (e.g. NaCl) layer (60), and gas-permeable membrane(70), such as a PVC/dioctyl adipate membrane comprisingtetradecylguanidinium chloride as the anion exchanger. FIG. 8 shows atop view of a PCB dissolved oxygen sensor (without the optional guardring) having a gold cathode (100), silver/silver chloride reversibleanode (110), and electrical contacts (120).

Briefly, the printed circuit board (PCB) sensor was produced fromcomputer Gerber plots using gold plated traces and pads. The PCB sensorwas also selectively plated with silver. It was then immersed in a saltsolution (NaCl) and a potential of 3.5V applied to plate a small amountof silver chloride on the surface of one electrode. A small amount ofelectrolyte (5 μL) was placed drop-wise by syringe over the twoelectrodes and allowed to dry completely. Next, a thin membrane layerwas cast over the entire sensor and allowed to polymerize. Once themembrane dried the sensor was placed in a thermostatically controlledwater chamber that was open to the atmosphere and allowed to hydrate.Once hydration was complete the unit stabilized and was is ready to beused.

A calibration curve for the PCB sensor is shown in FIG. 9 and shows thatit has a linear response to oxygen concentration. FIG. 10 shows acomparison of the response characteristics of a commercial Clark cellbased unit and the PCB sensor according to the disclosure. The sensorshows excellent stability and lifetimes, far exceeding those attainablefor a Clark cell of equivalent volume. A working embodiment with a 5 μLelectrolyte volume was stable and functional for one month of continuoususe. In fact, the commercial unit, with an electrolyte volume of about 1mL, that was used to provide calibration curves proved far less stableover the same period of time. Response times vary with membranethickness and easily matched those of a commercial unit testedalongside.

As discussed before, a number of different electrode arrangements may beproduced. For example, the PCB oxygen sensors also may be used to formsensor arrays, such as including sensor pads for other sensors, such asare useful for pH and temperature measurements.

EXAMPLE 4 Alternative Construction Techniques for PCB Sensors

Other construction techniques have been developed to help ensure a goodseal and prevent membrane lifting from the PCB surface.

In one embodiment, the sensor was constructed using the followingmethod:

-   -   1) a clean set of PCB sensor electrodes was first cleaned with        acetone and allowed to dry.    -   2) a section of double sided laminating material (3M, St. Paul,        Minn.) was cut and provided with a small (˜10 mm) hole.    -   3) The laminating material was placed on the PCB sensor such        that the hole was over the electrode portion of the sensor.    -   4) A small amount of electrolyte (NaCL, ˜5 uL) was placed        dropwise onto the centre of the electode area, inside the        laminating adhesive cut-out.    -   5) The electrolyte was allowed to dry.    -   6) A small piece of microporous cellulose acetate, cut to size,        was placed over the electrode area and pressed firmly into the        adhesive.    -   7) The PVC membrane material containing the ion exchanger        dissolved in a volatile solvent was added dropwise onto the        cellulose acetate material and allowed to dry.    -   8) The resulting sensor was then hydrated and “conditioned” by        applying a voltage of −0.5V until stabilized.    -   9) The sensor was then calibrated.

Sensors using this construction method have been tested for over 30 dayswith excellent results. These same sensors have been allowed to dry outand then rehydrated, with little degradation in performance.

In another embodiment, a method for helping to ensure a good seal andprevent membrane lifting from the PCB surface included providing anelectrolyte in a form of a hydrogel. Suitable hydrogels include but arenot restricted to a hydrogel selected from the following: cross-linkedacrylates, methyl methacrylates, methacrylates, hyroxyalkyl acrylates,hydroxyalkyl(meth)acrylates and acrylamides, silicone hydrogels,gelatin, cellulose nitrate, cellulose, agar and agarose. The followingmethod was used to produce a PCB sensor using a hydrogel:

-   -   1) A clean set of PCB sensor electrodes was first cleaned with        acetone and allowed to dry.    -   2) A section of double sided laminating material (3M, St. Paul,        Minn.) was cut and provided with a small (˜10 mm) hole.    -   3) The laminating material was placed on the PCB sensor such        that the hole was over the electrode portion of the sensor.    -   4) A small amount of agar (made with 1M NaCl) heated to a liquid        state was placed dropwise onto the centre of the electode area,        inside the laminating adhesive cut-out.    -   5) A thin plastic squeegie was used to draw a thin film of the        agar mixture across the cut-out.    -   6) A small piece of microporous cellulose acetate, cut to size,        was placed over the electrode area and pressed firmly into the        adhesive.    -   7) The PVC membrane material containing the ion exchanger        dissolved in a volatile solvent was added dropwise onto the        cellulose acetate material and allowed to dry.    -   8) The resulting sensor was then directly ready for        “conditioning” by applying a voltage of −0.5V until stabilized.    -   9) The sensor was then calibrated.        Advantageously, sensors prepared in this manner can be used        immediately after construction.

It should be recognized that the illustrated embodiments are onlyparticular examples of the inventions and should not be taken as alimitation on the scope of the inventions. Rather, the inventionsinclude all that comes within the scope an spirit of the followingclaims.

1. A method of preparing a gas permeable membrane comprising aguanidinium salt, the method comprising preparing a solvent comprisingthe guanidinium salt and imbibing a porous support polymer with thesolvent.
 2. The method of claim 1, wherein the membrane comprises asupported liquid membrane.
 3. The method of claim 1, wherein the poroussupport polymer comprises polytetrafluoroethylene.
 4. The method ofclaim 1, wherein the porous support polymer comprises poly(vinylchloride).
 5. The method of claim 4, further comprising plasticizing thepolymer for use as the membrane.
 6. The method of claim 4, furthercomprising plasticizing the polymer, wherein the polymer is highmolecular weight poly(vinyl chloride) and plasticizing is effected witha solvent selected from the group consisting adipate esters, sebacateesters, phthalate esters, glycol esters, low volatility ethers,trimellitic acid esters, phosphate triesters, chlorinated paraffins, andmixtures thereof.
 7. The method of claim 1, wherein the solventcomprises from 0.1% to 10% by weight of the guanidinium salt.
 8. Themethod of claim 7 further defined as preparing a solvent comprising 1%to 5% by weight guanidinium salt.
 9. The method of claim 1, wherein theguanidinium salt has the formula:

wherein R₁, R₂, R₃, R₄, R₅, and R₆ are independently selected from thegroup consisting of hydrogen, alkyl, substituted alkyl, cycloalkyl,substituted cycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl,substituted cycloalkenyl, alkynyl, aryl, substituted aryl, heteroaryland substituted heteroaryl, and X⁻ is an anion.
 10. The method of claim9, wherein R₁, R₂, R₃, R₄, R₅, and R₆ are independently selected fromthe group consisting of hydrogen, C1-30 alkyl, and aryl, and X⁻ isselected from the group consisting of chloride, bromide, fluoride,iodide, hydroxide, acetate, carbonate, sulfate and nitrate andcombinations thereof.
 11. A method of preparing an amperometric gassensor, the method comprising selecting a guanidinium salt, preparing asolvent containing the guanidinium salt, imbibing a gas permeablemembrane with the solvent, forming a reversible anode and an inertcathode, applying an electrolyte solution over the anode and thecathode, allowing the solution to evaporate and covering both electrodeswith the gas permeable membrane, such that the membrane preventscommunication between the electrolyte and an ambient environment exceptthrough the membrane.
 12. The method of claim 11, wherein the inertcathode is selected from the group consisting of gold, platinum, silver,palladium, iridium, rhodium, ruthenium, osmium and alloys thereof andthe reversible anode is selected from the group consisting ofsilver/silver halide, lead/lead sulfate, sliver/silver oxide-hydroxideand lead/lead oxide-hydroxide.
 13. The method of claim 12, wherein theinert cathode is gold and the reversible anode is a silver/silverchloride electrode.
 14. The method of claim sensor of claim 11, whereinthe gas permeable membrane is a supported liquid membrane.
 15. Themethod of claim 11, further comprising plasticizing a polymer to formthe membrane.
 16. A method of measuring dissolved gas in a liquid samplethe method comprising removing an ionic product of an electrode reactionfrom an electrolyte volume using a guanidinium salt and transporting anion consumed in the electrode reaction from the liquid sample into theelectrolyte volume.
 17. The method of claim 16 wherein the ionic productis hydroxide ion and the ion consumed in the electrode reaction ischloride.
 18. The method of claim 16, wherein the liquid samplecomprises seawater, a biological fluid or a foodstuff.
 19. A solid stateamperometric gas sensor, comprising: a gas-permeable membrane comprisinga guanidinium salt; an electrolyte salt in contact with thegas-permeable liquid membrane through a first surface of the electrolytesalt; and a pair of electrodes in contact with a second, oppositesurface of the electrolyte salt.
 20. The sensor of claim 19 where thegas-permeable membrane is a supported liquid membrane.
 21. The sensor ofclaim 19 where the gas-permeable membrane comprises a plasticizedpolymer.
 22. The sensor of claim 19 where the electrolyte salt isselected from the group consisting of Group I metal halides.
 23. Thesensor of claim 22 where the electrolyte salt is selected from KCl andNaCl and mixtures thereof.
 24. A dissolved oxygen sensor, comprising: agas permeable membrane, other than a supported liquid membrane, themembrane comprising a guanidinium salt; an inert cathode; and areversible anode.
 25. The dissolved oxygen sensor of claim 24 furthercomprising an electrolyte where the cathode and anode are imprinted onan oxygen impervious, insulating substrate, the electrolyte is incontact with the cathode and anode and the gas-permeable membraneprevents communication between the electrolyte and an external medium.26. A gas permeable membrane comprising a guanidinium salt, and aplasticized polymer, wherein the plasticized polymer is high molecularweight poly(vinyl chloride) plasticized with a solvent selected from thegroup consisting adipate esters, sebacate esters, phthalate esters,glycol esters, low volatility ethers, trimellitic acid esters, phosphatetriesters, chlorinated paraffins, and mixtures thereof.
 27. The membraneof claim 26 comprising a supported liquid membrane.
 28. The membrane ofclaim 26 comprising from 0.1% to 10% by weight of the guanidinium salt.29. The membrane of claim 26 comprising 1% to 5% by weight of theguanidinium salt.
 30. The membrane of claim 26, wherein the guanidiniumsalt has the formula:

wherein R₁, R₂, R₃, R₄, R₅ and R₆ are independently selected from thegroup consisting of substituted alkyl, cycloalkyl, substitutedcycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substitutedcycloalkenyl, alkynyl, substituted aryl, heteroaryl and substitutedheteroaryl, and X⁻ is an anion.
 31. The gas permeable membrane of claim30, wherein X⁻ is selected from the group consisting of chloride,bromide, fluoride, iodide, hydroxide, acetate, carbonate, sulfate andnitrate and combinations thereof.
 32. The gas permeable membrane ofclaim 30, wherein R₁, R₂, R₃, R₄, R₅ and R₆ are independently selectedfrom the group consisting of hydrogen, C1-30 alkyl, and aryl, and X⁻ isselected from the group consisting of chloride, bromide, fluoride,iodide, hydroxide, acetate, carbonate, sulfate and nitrate andcombinations thereof.
 33. An amperometric gas sensor comprising agas-permeable membrane comprising a guanidinium salt, an inert cathodeand a reversible anode, the inert cathode selected from the groupconsisting of silver, palladium, iridium, rhodium, ruthenium, and osmiumand alloys thereof and the reversible anode is selected from the groupconsisting of lead/lead sulfate, sliver/silver oxide-hydroxide andlead/lead oxide-hydroxide.
 34. The sensor of claim 33, wherein the gaspermeable membrane comprises a supported liquid membrane.
 35. The sensorof claim 34, wherein the gas permeable membrane comprises a plasticizedpolymer.
 36. The sensor of claim 34, wherein the guanidinium saltremoves an ionic product of an electrode reaction from an electrolytevolume and transports an ion consumed in the electrode reaction from thesample into the electrolyte volume.
 37. The sensor of claim 36, whereinthe ionic product is hydroxide ion and the ion consumed in the electrodereaction is chloride.
 38. A method of preparing an amperometric gassensor, the method comprising selecting a guanidinium salt, preparing asolvent containing the guanidinium salt, imbibing a gas permeablemembrane with the solvent, forming a reversible anode and a suitableinert cathode, applying an electrolyte layer to and covering bothelectrodes with the gas permeable membrane, such that the membraneprevents communication between the outer electrolyte and an ambientenvironment.
 39. The method of claim 38, wherein the electrolyte layeris a hydrogel.
 40. The method of claim 39, wherein the hydrogel isselected from the group consisting of gelatin, cellulose nitrate,cellulose, agar and agarose.
 41. The method of claim 39 wherein thehydrogel is selected from the group consisting of cross-linkedacrylates, methyl methacrylates, methacrylates, hyroxyalkyl acrylates,hydroxyalkyl(meth)acrylates and acrylamides.
 42. The sensor of claim 19,wherein the electrolyte salt comprises a hydrogel.
 43. The sensor ofclaim 42 wherein the bydrogel is selected from the group consisting ofcross-linked acrylates, methyl methacrylates, methacrylates, hyroxyalkylacrylates, hydroxyalkyl(meth)acrylates and acrylamides.
 44. Anamperometric gas sensor, comprising; an anode and a cathode deposited ona gas-impervious substrate; an electrolyte between the anode and thecathode, and a gas-permeable membrane comprising a guanidinium saltcovering the anode, the cathode and the electrolyte.
 45. The sensor ofclaim 44, further comprising a well surrounding the anode and thecathode, wherein the gas permeable membrane covers the well and definesan electrolyte volume within the well.
 46. The sensor of claim 45,wherein the well defining the electrolyte volume comprises a laminatematerial having a hole, wherein the hole is placed over the anode andthe cathode.
 47. The sensor of claim 44, further comprising a guard ringdeposited on the gas-impervious substrate, wherein the gas permeablemembrane also covers the guard ring.
 48. The sensor of claim 44, whereinthe electrolyte comprises a solid salt.
 49. The sensor of claim 44,wherein the electrolyte comprises a hydrogel.
 50. The sensor of claim44, wherein the gas-permeable membrane comprises a supported liquidmembrane.
 51. The sensor of claim 50, wherein the supported liquidmembrane comprises a porous support polymer comprising a solvent. 52.The sensor of claim 51, wherein the porous support polymer comprises apolytetrafluoroethylene membrane and the solvent is selected from thegroup consisting of o-nitrophenyl octyl ether and dioctyl adipate, andmixtures thereof.
 53. The sensor of claim 50, wherein the supportedliquid membrane comprises a plasticized polymer.
 54. The sensor of claim53, wherein the plasticized polymer comprises poly(vinyl chloride) and aphthalate plasticizer.
 55. The sensor of claim 54, wherein theplasticized polymer comprises a high molecular weight poly(vinylchloride) plasticized with a solvent selected from the group consistingof o-nitrophenyl octyl ether and dioctyl adipate, and mixtures thereof.56. The sensor of claim 44, wherein the guanidinium salt is notcovalently bonded to the membrane.
 57. The sensor of claim 44, whereinthe electrolyte comprises a Group I metal halide.
 58. The sensor ofclaim 57, wherein the Group I metal halide comprises KCl, NaCl or amixture thereof.
 59. The sensor of claim 44, wherein the guanidiniumsalt comprises 1% to 5% by weight of the membrane.
 60. The sensor ofclaim 44, wherein the guanidinium salt has the formula:

wherein R₁, R₂, R₃, R₄, R₅ and R₆ are independently selected from thegroup consisting of substituted alkyl, cycloalkyl, substitutedcycloalkyl, alkenyl, substituted alkenyl, cycloalkenyl, substitutedcycloalkenyl, alkynyl, substituted aryl, heteroaryl and substitutedheteroaryl such that the salt has an affinity for the membrane and X⁻ isan anion.
 61. The sensor of claim 60, wherein X⁻ is selected from thegroup consisting of chloride, bromide, fluoride, iodide, hydroxide,acetate, carbonate, sulfate and nitrate and combinations thereof. 62.The sensor of claim 60, wherein R₁, R₂, R₃, R₄, R₅ and R₆ areindependently selected from the group consisting of hydrogen, C1-30alkyl, and aryl.
 63. The sensor of claim 49, wherein the hydrogel isselected from the group consisting of cross-linked acrylates, methylmethacrylates, methacrylates, hydryxalkyl acrylates,hydroxyalkyl(meth)acrylates, acrylamides, silicone hydrogels, gelatin,cellulose nitrate, cellulose, agar, and agarose and combinationsthereof.
 64. A method of making an amperometric gas sensor, comprising:depositing an anode and a cathode onto a gas-impervious substrate;placing an electrolyte between the anode and the cathode; and coveringthe anode, the cathode and the electrolyte with a gas-permeablemembrane, wherein the gas-permeable membrane comprises a guanidiniumsalt.
 65. The method of claim 64, wherein depositing comprises printingthe anode and cathode onto the substrate using a method for printingcircuit boards.
 66. The method of claim 64 further comprising depositinga guard ring onto the substrate.
 67. The method of claim 64 furthercomprising forming a well around the anode and the cathode and coveringthe well with the membrane to define an electrolyte volume.
 68. Themethod of claim 67, wherein placing an electrolyte between the anode andthe cathode comprises adding the electrolyte to the well.
 69. The methodof claim 68, wherein adding the electrolyte to the well comprises addingthe electrolyte to the well as a solution.
 70. The method of claim 69,wherein the solution is allowed to dry.
 71. The method of claim 68,wherein adding the electrolyte to the well comprises forming a hydrogelin the well.
 72. The method of claim 71, wherein the hydrogel isselected from the group consisting of cross-linked acrylates, methylmethacrylates, methacrylates, hydryxalkyl acrylates,hydroxyalkyl(meth)acrylates, acrylamides, silicone hydrogels, gelatin,cellulose nitrate, cellulose, agar, and agarose and combinationsthereof.
 73. The method of claim 67, wherein forming a well around theanode and the cathode comprises placing a laminating material comprisinga hole onto the substrate such that the hole is disposed over the anodeand cathode.
 74. The method of claim 67, wherein covering the well withthe membrane comprises depositing a plasticized PVC membrane materialdissolved in a volatile solvent over the well.
 75. The method of claim74, further comprising first covering the well with a layer ofmicroporous cellulose acetate and then depositing the PVC membranematerial onto the microporous cellulose acetate.
 76. An amperometric gassensor formed on a printed circuit board, comprising: an inert cathodeand a reversible anode patterned on a gas-imperviouselectrically-insulating substrate; an electrolyte between the cathodeand the anode; and a gas-permeable membrane covering the cathode, theanode and the electrolyte; wherein the membrane is sealed at its outeredges to prevent communication between the electrolyte and a medium inwhich a gas is sensed except through the membrane.
 77. An amperometricgas sensor, comprising: an inert cathode and a reversible anode printedon a gas-impervious circuit board substrate; a well surrounding thecathode and the anode; a hydrogel in the well, wherein the hydrogelcomprises an electrolyte; and a gas-permeable membrane covering thewell, wherein the gas permeable membrane comprises a guanidinium salt.78. The sensor of claim 77, wherein the inert cathode is selected fromthe group consisting of gold, platinum, silver, palladium, iridium,rhodium, ruthenium and osmium, and alloys thereof; and the reversibleanode comprises a material selected from the group consisting ofsilver/silver halide, lead/lead sulfate, silver/silver oxide hydroxideand lead/lead oxide-hydroxide.
 79. The sensor of claim 78, wherein theinert cathode comprises a material selected from the group comprising ofgold and platinum and the reversible anode is an Ag/AgCl electrode. 80.The sensor of claim 77, wherein the hydrogel is selected from the groupconsisting of cross-linked acrylates, methyl methacrylates,methacrylates, hydryxalkyl acrylates, hydroxyalkyl(meth)acrylates,acrylamides, silicone hydrogels, gelatin, cellulose nitrate, cellulose,agar, and agarose, and combinations thereof.